High-density information storage, such as magnetic recording and topographic recording on compact discs, is an important part of modem computer technology. Conventional magnetic recording systems, such as computer hard disk drives, typically use a continuous magnetic thin film on a rigid substrate as the recording medium. Each bit of information is stored by magnetizing a small area on the thin magnetic film using a write head that provides a suitable writing magnetic field. The magnetization strength and the location of each magnetic bit must be defined precisely to allow a flying read head (sensitive magnetic field sensor) to retrieve the written information. Each magnetic bit in the magnetic recording media contains one magnetized region consisting of many small magnetized grains. Because of the trend toward higher recording density, the magnetic bit size is continuously being reduced to a point approaching superparamagnetic limits of magnetic recording.
In order to overcome superparamagnetic limits of magnetic recording, patterned magnetic media with discrete magnetic regions have been proposed, for example, see U.S. Pat. No. 5,820,769 to Chou et al., U.S. Pat. No. 5,5587,223 to White et al., U.S. Pat. No. 6,440,520 B1 to Baglin et al., and co-pending U.S. patent application Ser. No. 10/262,462, “Ultra-high-density information storage medium and method for making the same”, by S. Jin, filed Sep. 30, 2002, now abandoned, and corresponding divisional application Ser. No. 11/020.286, filed Dec. 8, 2004. Another approach of reducing the superparamagnetic instability problem is to use a magnetic recording media with antiferromagnetically coupled ferromagnetic films separated by a very thin layer of non-magnetic spacer material, such as described in U.S. Pat. No. 6,280,813 to Carey, et al. issued Aug. 28, 2001.
Conventional magnetic read/write heads are generally based on magneto-resistive (MR) sensors or giant magneto-resistant (GMR) sensors. See U.S. Pat. No. 6,081,408 issued to Partee et al., Jun. 27, 2000; U.S. Pat. No. 5,668,689 issued to Schultz et al., Sep. 16, 1997; U.S. Pat. No. 5,159,513 issued to Dieny et al., Oct. 27, 1992; U.S. Pat. No. 6,356,420 issued to Everitt et al., Mar. 12, 2002; and U.S. Pat. No. 5,576,914 issued to Rottmayer et al., Nov. 19, 1996. While MR sensors and GMR sensors have been adequate for reading of magnetic bits in today's recording media, they are not likely to be sensitive enough for future ultra-high-density recording media (≧200 gigabits per square inch). As the recording density is substantially raised with significantly reduced magnetic bit size, e.g., by one to two orders of magnitude, the magnetic field signal from each of the recorded bits is substantially reduced. Therefore, there is a need to develop a highly sensitive magnetic read head.
Another type of recording media for mass information storage is compact disc (CD). The CDs have been used mostly for read-only memory (ROM) applications, although a rapid progress is being made in the use of writeable CD disc memory technology. A CD is usually made of ˜1 mm thick plastic, coated with an aluminum layer and a protective plastic coating. Information is topographically recorded as microscopic bumps or recessed holes arranged as a single or continuous spiral track of data. As the CD disc is rotated in the CD player, a laser beam, focused by a lens system, follows the track and reads the presence or absence of the bumps.
As the bit size in the current CDs is typically larger than about 200 nm, the recording density is less than a few gigabits per square inch. With the advance of information storage density in compact disc media, such as described in the above incorporated co-pending application Ser. No. 11/020,286, the information bit size of bumps or recessed holes can be extremely fine, for example, of the order of 10 to 50 nm in diameter, giving rise to a recording density of about 40 gigabits to 1 terabits per square inch. The laser optical technique can no longer effectively detect such fine nanoscale features which are well below the wavelengths of the laser beam. Therefore, there is a need to develop new techniques for reading of such nanoscale information bits on ultra-high-density CD discs.